Triple negative breast cancer (TNBC) is an orphan disease that attacks 46,000 US women every year. TNBC cells lack human estrogen receptor, progesterone receptor, and epidermal growth factor receptor 2 (Her2), the targets of existing medicines. TNBC recurs after standard-of-care chemotherapy and radiation, killing its victims within 4 years. New promising therapies such as poly(ADP-ribose) polymerase (PARP) inhibitors only benefit a small subset of patients with BRCA1/2 mutations. Thus, TNBC shows a critical need for molecularly-targeted therapy. Most TNBC cells and associated stroma show high microRNA 21 (miR-21), which decreases tumor suppressor proteins that keep cell growth in check. A molecularly-targeted therapeutic to block miR-21 in TNBC is our objective. Premise: We have designed a strong miR-21 blocking agent using aminomethyl bridged nucleic acid (BNA), with strong basepairing, Tm >80C, low toxicity, and serum stability, conjugated to a peptide ligand for endocytosis by the insulin-like growth factor 1 receptor (IGF1R). 42% of TNBC tumors show constitutive IGF1R signaling. The peptide ligand for IGF1R provides a unique strategy for delivering miR-21 blocker specifically into the TNBC cells. Our agent basepairs with miR-21 in the RNA- induced silencing complex (RISC), freeing target mRNAs from miR-21 attack. Our miR-21 BNA elevated tumor suppressor proteins, suppressed immune checkpoint gene expression, increased apoptosis, slowed proliferation and migration in multiple TNBC lines. BNA-peptide in sterile saline can be administered by infusion. Our strategy optimizes cancer cell-specific delivery to block proliferation and immune checkpoints, covered by a pending PCT patent application, licensed by Bound Therapeutics LLC. Hypothesis: Our unique design for short microRNA blockers conjugated to a receptor ligand will direct TNBC cell uptake and slow the growth of TNBC orthotopic xenografts with minimal toxicity. Aim 1: Measure the effects of the lead miR-21 blocker on proliferation, apoptosis, invasion, and cellular expression of miR-21 target mRNAs and checkpoint proteins in 5 molecular subtypes of TNBC cells. Measure tumor response and immune activation by the lead miR-21 blocker in TNBC 4T1-luciferase orthotopic xenografts in immunocompetent syngeneic mice. Predicted results: Significant inhibition of tumor growth and immune checkpoints, and elevation of T-cell response. Aim 2: Measure toxicity of the lead miR-21 blocker in human hepatocytes by transcriptome analysis. Measure toxicity of the lead miR-21 blocker in mice by liver and kidney serum markers and weight. Measure on-target and off-target transcriptome effects, pharmacokinetics, and biodistribution of the lead miR-21 blocker in the mouse model. Predicted results: Minimal toxicity to human hepatocytes and murine host cells. Impact: We seek proof-of-concept to derisk the commercialization of a BNA-peptide TNBC therapeutic, enabling a full preclinical study of potency, T-cell response, toxicology, and pharmacokinetics, prior to IND and a Phase I single agent safety trial. We expect that miRNA blockade will significantly increase TNBC patient survival.